3 files changed, 22 insertions, 34 deletions
diff --git a/src/or/dns.c b/src/or/dns.c
index 5f9813b912..41a6dfd0a4 100644
--- a/src/or/dns.c
+++ b/src/or/dns.c
@@ -243,29 +243,19 @@ has_dns_init_failed(void)
 }
 
 /** Helper: Given a TTL from a DNS response, determine what TTL to give the
- * OP that asked us to resolve it. */
+ * OP that asked us to resolve it, and how long to cache that record
+ * ourselves. */
 uint32_t
 dns_clip_ttl(uint32_t ttl)
 {
-  if (ttl < MIN_DNS_TTL)
-    return MIN_DNS_TTL;
-  else if (ttl > MAX_DNS_TTL)
-    return MAX_DNS_TTL;
-  else
-    return ttl;
-}
-
-/** Helper: Given a TTL from a DNS response, determine how long to hold it in
- * our cache. */
-STATIC uint32_t
-dns_get_expiry_ttl(uint32_t ttl)
-{
-  if (ttl < MIN_DNS_TTL)
-    return MIN_DNS_TTL;
-  else if (ttl > MAX_DNS_ENTRY_AGE)
-    return MAX_DNS_ENTRY_AGE;
+  /* This logic is a defense against "DefectTor" DNS-based traffic
+   * confirmation attacks, as in https://nymity.ch/tor-dns/tor-dns.pdf .
+   * We only give two values: a "low" value and a "high" value.
+   */
+  if (ttl < MIN_DNS_TTL_AT_EXIT)
+    return MIN_DNS_TTL_AT_EXIT;
   else
-    return ttl;
+    return MAX_DNS_TTL_AT_EXIT;
 }
 
 /** Helper: free storage held by an entry in the DNS cache. */
@@ -1317,7 +1307,7 @@ make_pending_resolve_cached(cached_resolve_t *resolve)
         resolve->ttl_hostname < ttl)
       ttl = resolve->ttl_hostname;
 
-    set_expiry(new_resolve, time(NULL) + dns_get_expiry_ttl(ttl));
+    set_expiry(new_resolve, time(NULL) + dns_clip_ttl(ttl));
   }
 
   assert_cache_ok();
diff --git a/src/or/dns.h b/src/or/dns.h
index b14f7dd29c..951a2a3467 100644
--- a/src/or/dns.h
+++ b/src/or/dns.h
@@ -12,6 +12,18 @@
 #ifndef TOR_DNS_H
 #define TOR_DNS_H
 
+/** Lowest value for DNS ttl that a server will give. */
+#define MIN_DNS_TTL_AT_EXIT (5*60)
+/** Highest value for DNS ttl that a server will give. */
+#define MAX_DNS_TTL_AT_EXIT (60*60)
+
+/** How long do we keep DNS cache entries before purging them (regardless of
+ * their TTL)? */
+#define MAX_DNS_ENTRY_AGE (3*60*60)
+/** How long do we cache/tell clients to cache DNS records when no TTL is
+ * known? */
+#define DEFAULT_DNS_TTL (30*60)
+
 int dns_init(void);
 int has_dns_init_failed(void);
 void dns_free_all(void);
@@ -31,8 +43,6 @@ void dump_dns_mem_usage(int severity);
 #ifdef DNS_PRIVATE
 #include "dns_structs.h"
 
-STATIC uint32_t dns_get_expiry_ttl(uint32_t ttl);
-
 MOCK_DECL(STATIC int,dns_resolve_impl,(edge_connection_t *exitconn,
 int is_resolve,or_circuit_t *oncirc, char **hostname_out,
 int *made_connection_pending_out, cached_resolve_t **resolve_out));
diff --git a/src/or/or.h b/src/or/or.h
index 66717792b4..e48cd92f03 100644
--- a/src/or/or.h
+++ b/src/or/or.h
@@ -143,18 +143,6 @@
 /** Maximum size of a single extrainfo document, as above. */
 #define MAX_EXTRAINFO_UPLOAD_SIZE 50000
 
-/** How long do we keep DNS cache entries before purging them (regardless of
- * their TTL)? */
-#define MAX_DNS_ENTRY_AGE (30*60)
-/** How long do we cache/tell clients to cache DNS records when no TTL is
- * known? */
-#define DEFAULT_DNS_TTL (30*60)
-/** How long can a TTL be before we stop believing it? */
-#define MAX_DNS_TTL (3*60*60)
-/** How small can a TTL be before we stop believing it?  Provides rudimentary
- * pinning. */
-#define MIN_DNS_TTL 60
-
 /** How often do we rotate onion keys? */
 #define MIN_ONION_KEY_LIFETIME (7*24*60*60)
 /** How often do we rotate TLS contexts? */